1. Field of the Invention
The present invention relates to a process for preparing a substrate of an optical disc. In particular, the present invention relates to a process for a substrate of an optical disc having highly precise track pitches and formating positions.
2. Description of Prior Art
Information recording media utilizing a beam of high energy density such as a laser beam have been developed in recent years and are now put into practical use. Such recording medium is generally called "optical disc", and the practical applications thereof have been found, for example, as a video disc and an audio disc as well as a disc memory for a large-capacity computer and a large-capacity static image file. The optical disc is generally prepared utilizing a metallic stamper formed by use of a master disc in which a groove or a series of pits (a extremely small sized pattern in the form of depression) corresponding to information signals is formed in the form of a spiral or circles. In detail, the substrate of the optical disc having the above-mentioned groove or the series of pits is produced by molding a material for the substrate using the stamper as a mold, and on the substrate are optionally formed a recording layer, reflecting layer, protective layer, etc., whereby an optical disc is prepared.
The substrate of the optical disc used in writable-type optical disc has a groove (e.g., a single groove or a set of plural grooves in the form of a spiral or circles) serving as a tracking guide, and/or a series of pits. The substrate having the groove is called "replica disc". The substrate of the optical disc only for reproduction has generally a series of pits corresponding to recorded signals.
The above-mentioned master disc, which is used for preparing an optical disc, is prepared by the following processes. In the first place, a photoresist layer of a positive type is formed on an appropriate substrate such as a glass plate to form a resist disc. Then, the photo-resist layer is irradiated with (i.e., exposed to) a laser beam under rotation of the resist disc at a predetermined velocity to form a latent image of a groove or a series of pits (preparation of a recorded resist disc). After the irradiation is complete, the photo-resist layer is subjected to developing treatment to remove the irradiated portion of the layer and then subjected to baking treatment for a certain period of time to cure the remaining portion of the photoresist layer. Thus, a master disc having a groove or a series of pits thereon is prepared.
The irradiation of the photoresist layer with the laser beam (which is called "laser cutting process" or "cutting procedure") is generally performed using a laser cutting machine as shown in FIG. 1. In the cutting procedure, a recording head of emitting a laser beam moves generally from an inner side to an outer side on the resist disc under rotation, for instance, at 1,800 r.p.m., whereby signals are recorded (cut) on the resit disc (e.g., recording of one field of NTSC video signal on one track) as a latent pattern image. For example, in the case that on the resist disc are spirally formed tracks of 20,000 at a track pitch of 1.6 .mu.m within an area from a radius of 29.0 mm to that of 61.0 mm, the cutting is started simultaneously when the recording head reaches the position far from center of the resist disc at 29.0 mm, the cutting being conducted so as to control the moving velocity of the recording head to 2.88 mm/min. (1.6.times.10.sup.-3 .times.1800=2.88), and the cutting is terminated when the resist disc revolves as much as 20,000 times. The resist disc on which the latent pattern image is recorded as above is developed to obtain a master disc.
The stamper is produced by processes of forming a conductive area on the surface of the master disc to change its surface into one having conductivity, electroplating a metal on the surface, separating both the elecroplated metal layer and the photoresist layer from the glass disc, and removing the photoresist layer from the elecroplated metal layer. This metal layer is used as a stamper. The surface of the obtained stamper has a reverse pattern of that of the master disc. The pattern of the stamper has the same dimension as that of the master disc except that it has the reverse pattern. The substrate of the optical disc is generally produced by molding a resin by use of an injection mold equiped with the stamper as obtained above. Thus produced substrate has a surface of the same pattern as that of the master disc.
Thus, the pattern such as a groove or a series of pits on the substrate of the optical disc should correspond to that of the master disc.
The molding of a resin (i.e., plastic) using the stamper is generally conducted at high temperatures (about 100.degree. C. in an area of contact between the stamper and the molten resin) for obtaining a substrate of an optical disc. The molded resin inherently shrinks in the course that it is cooled to an ambient temperature. The stamper made of metal such as Ni also shrinks in the same course. A coefficient of linear expansion of the plastic is generally larger than that of the metal. Accordingly, the diameter of the molded substrate is made smaller than that of the stamper, and the pattern of the groove or the series of pits which are spirally or concentrically formed on the substrate of the optical disc is also smaller in diameter than that of the stamper.
Thus, the inner diameter, the outer diameter and the pattern such as the groove of the stamper are not accurately copied to the substrate of the optical disc. Therefore, it is observed that erroneous operation at the start of reproduction occurs due to ill location of the startig position for reproduction in the inner side (starting position of tracking) and tracking error occurrs owing to inaccurate pattern such as inaccurate groove position. Further, erroneous operation is observed at the time of start of recording, the starting position for recording is same as that of the reproduction and tracking error in recording procedure occurrs in the same manner as in the above reproduction.
In order to avoid such troubles, a substrate of an optical disc is generally molded using a stamper which has a pattern whose starting site is located at a position outer than the predetermined position. In consideration of both the coefficient of linear expansion of the resin of the substrate and that of the metal of the stamper. The starting position of reproduction (recording) in the inner side is formed at the predetermined position on the resulting substrate, so that occurrence of erroneous operation at the start of reproduction decreases extremely. However, tracking error and read error in reproducing or recording procedure are not satisfactorily prevented.